Recently, there have been dramatic changes in the telecommunications industry. For example, deregulation of local markets resulted in the emergence of new technologies in this industry. Furthermore, a growing demand for Internet access sparked development of new technologies that deliver high speed data services using existing infrastructure.
As is well known in the industry, Digital Subscriber Line, or DSL, is one of the most promising new technologies for delivering superior service and high speed connections using the existing infrastructure. DSL service is implemented in several different ways, such as asymmetrical DSL, or ADSL, where upstream and downstream have different bandwidths, symmetrical DSL, or SDSL, where upstream and downstream have the same bandwidth. In general, these DSL services use the existing copper loop that is used for conventional telephony but provide much higher bandwidth. However, to achieve such high data rates, DSL services operate at higher frequencies and are thus more sensitive to the length and quality of the copper loop. So-called plain old telephone service (POTS) lines were originally designed for voice communications which cover a limited frequency bandwidth of about 4 KHz. As a result, a POTS line that works well transmitting voice signals might not work well for data signals. Therefore, sophisticated testing is needed to find out which lines are not suitable for data transmission and why particular lines are unable to support DSL data transmission.
In order to test the data transmission through a POTS line noise signals simulating cross talk impulses and RF ingress are injected into the line under test. The noise signals need to be injected at an appropriate voltage level without disturbing the impedance of the line under test, or more specific, the test loop. In particular, differential mode noise signals need to be injected at high impedance while common mode noise signals need to be injected at low impedance. In prior art systems noise in injected at an end of a line, via resistors having a large resistance for differential mode noise signals or via resistors having a low resistance for common mode noise signals. Injecting a differential noise signal using such a system requires high supply voltages and an analog circuitry for providing such high voltages. Furthermore, it requires switching between two electrical circuits for injecting a noise signal in differential mode and in common mode. An alternative system utilizes inductive coupling by inserting transformer coils into the line in order to overcome the drawback of high supply voltages. However, in both prior art systems the line needs to be uncoupled at one end or split for injecting of the noise signal.